Method of repairing bolts with replaceable threads

ABSTRACT

A screw with replaceable threads is particularly useful to releasably join the spiders of rotational molding machines. The screw comprises a master bolt that passes through a first spider, and a tip that is removably assembled to the master bolt. The tip is threaded to selectively engage and disengage a receiver on the second spider. When the tip is assembled to the master bolt, the screw is captured in the first spider. When the tip has worn through use, it can be replaced without disturbing the master bolt. Similarly, when the tip seizes to the receiver, the tip and receiver are replaceable as a unit without disturbing the master bolt.

This is a divisional of U.S. patent application Ser. No. 08/752,974,filed on Dec. 2, 1996, now U.S. Pat. No. 5,807,589.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to fasteners, and more particularly to threadedfasteners in which the threads are replaceable on a shank.

2. Description of the Prior Art Threaded fasteners in the form of boltsand screws are well known and are in widespread use. Such fasteners areusually in one piece, and they invariably include a head and a threadedshank. In many instances, the shank is threaded for its full length. Inother cases, only a portion of the shank is threaded.

The industrial process of rotational molding is a particularly severeapplication for threaded fasteners. Large screws, which typically have a0.75 inch diameter and an eight-inch length, are used to releasably jointwo large mold supporting spiders to each other. The screw shanks passthrough clearance holes in a first spider. The screw threads, which areusually hardened, engage receivers in the other spider. At the start ofa molding cycle, the screws are tightened into their associatedreceivers to draw the spiders toward each other and close the molds. Atthe end of the cycle, the screws are reversed to completely disengagefrom the receivers. The spiders can then be separated from each otherand the molds opened. After the workpieces have been removed from themolds, the process is repeated. The repeated turning of the screwscauses their threads to wear relatively quickly, even though they arehardened. Consequently, the screws, which are expensive, mustperiodically be replaced. In addition, debris and coolant from themachinery can enter the receivers. As a result, the screw threadsoccasionally seize to the receivers. When that occurs, the screw shankmust be flame cut in two. The entire screw, as well as the receiver, arediscarded. That remedy is expensive, not only from the standpoint ofmaterial loss but also from the associated unproductive downtime.

A further problem with prior screws used in the rotational moldingprocess is that they tend to fall out of the first spider when thespiders are separated at the end of a molding cycle. The worker mustthen either search for the screw or obtain a new one.

In some other applications of threaded fasteners, it is desirable oreven necessary to employ multi-component fasteners. Such applicationsinclude railroad tracks; the bolt of U.S. Pat. No. 1,208,550 is anexample. Another application of multi-part fasteners are self-tappingscrews such as are shown in U.S. Pat. No. 4,022,099 and 4,900,207. U.S.patent 4,126,338 shows a housing end and replaceable threaded sleevethat are useful for coupling a fluid conduit. Canada patent 715,793describes a shear bolt that includes a stud that threads into a sleeve.

None of the fasteners of the foregoing patents is suitable for use withrotational molding equipment. Therefore, a need exists for improvementsin the spider joining screws that are used in rotational molding.

SUMMARY OF THE INVENTION

In accordance with the present invention, a screw with replaceablethreads is provided that greatly reduces the expenses associated withrotational molding processes. This is accomplished by apparatus thatincludes a master bolt in combination with a threaded tip.

The master bolt has a head and a shank. The shank is long enough to passthrough the particular mold supporting spider with which the screw ofthe invention is to be used. There is a bore in the shank end surface. Across hole is drilled transversely through the shank wall at the bore.The location of the cross hole is accurately controlled relative to theshank end surface.

The tip has a pilot on one end and threads on the other end. The pilotis sized to fit snugly in the master bolt bore. There is a shoulderbetween the pilot and the threads, and a cross hole in the pilot. Thedistance between the cross hole and the shoulder of the tip is the sameas the distance between the cross hole and shank end surface of themaster bolt. Consequently, when the tip pilot is inserted into themaster bolt bore until the tip shoulder abuts the shank end surface, thecross holes on the two parts lie in the same plane. The two parts arerotated relative to each other until the cross holes align. Then a pincan be inserted through the cross holes of the two parts.

In use, the master bolt shank is passed through a first spider. The tippilot is inserted into the shank bore, and the tip is pinned to themaster bolt. The pin is long enough to cooperate with the master bolthead to capture the master bolt on the first spider such that the masterbolt does not fall out of the first spider during operation of themolding equipment. The resulting multi-component screw is used ingenerally the same manner as the prior one-piece screws.

However, when the threads of the tip wear, it is necessary only toremove the pin to separate the tip from the master bolt, and only thetip need be discarded. A new tip is then inserted into and pinned to themaster bolt. Similarly, when a tip thread seizes to a receiver, the tipis unpinned from the master bolt, and the receiver is disconnected fromits spider. The tip and receiver are discarded as a unit. A new tip ispinned to the master bolt, and a new receiver is installed in thespider. Downtime of the molding machine during tip and spiderreplacement is substantially reduced compared with prior screws.

The method and apparatus of the invention, using a multi-componentscrew, thus greatly increases the productivity and economy of rotationalmolding machines. The threaded tip of the screw can be quickly replacedon the master bolt when necessary, even though the screw functions inthe same manner as conventional screws during normal operation of themolding equipment.

Other advantages, benefits, and features of the present invention willbecome apparent to those skilled in the art upon reading the detaileddescription of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a portion of a typicalrotational molding machine that employs the present invention.

FIG. 2 is a view on an enlarged scale taken along line 2--2 of FIG. 1.

FIG. 3 is an exploded front view of the screw with replaceable threadsaccording to the invention.

FIG. 4 is a side view of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Although the disclosure hereof is detailed and exact to enable thoseskilled in the art to practice the invention, the physical embodimentsherein disclosed merely exemplify the invention, which may be embodiedin other specific structure. The scope of the invention is defined inthe claims appended hereto.

Referring to FIGS. 1 and 2, reference numeral 1 indicates a portion of atypical rotational molding machine. The rotational molding machine 1 hasa top spider 3 and a bottom spider 5. The spiders 3 and 5 supportrespective halves 6 and 6A of large molds. In FIG. 1, two molds 6, 6Aare shown, but it will be understood that the invention is not limitedto a particular size or type of rotational molding machine.

In accordance with the present invention, and also looking at FIGS. 3and 4, a number of screws 11 with replaceable tips are used toreleasably join the spiders 3 and 5 to each other. Each screw withreplaceable tip 11 is comprised of a master bolt 13, a tip 15, and a pin17. The master bolt 13 is made as a cylindrical shank 19 having a head21 on one end thereof. The length of the master bolt shank 19 can varyto suit the spiders of different rotational molding machines 1.Preferably, the head 21 is a hex head. The other end 23 of the shank 19has a bore 25. A cross hole 27 is at an accurately located distance fromthe shank end 23. The distance from the cross hole 27 to the shank end23 is the same for all length shanks.

The tip 15 has a first end with a pilot 29 of slightly smaller diameterthan the diameter of the master bolt bore 25. The pilot 29 terminates ina shoulder 31. A cross hole 33 in the pilot has the same diameter as thecross hole 27 in the master bolt 13. The tip cross hole 33 is located atthe same distance from the shoulder 31 as the hole 27 is located fromthe master bolt end 23. The tip is threaded at 34 between the shoulderand a guide 37 at the tip second end. The guide 37 is shown as beingcylindrical in shape with a diameter less than the diameter of thethreads 34. However, it will be appreciated that the guide can have atapered surface, if desired.

The pin 17 is sized to fit snugly in the cross holes 27 and 23. In theillustrated construction, the pin is shown as a cylindrical pin with ahead 39 and a cotter pin 41. However, a hair pin or ball-and-detent pinalso are acceptable.

As best shown in FIG. 2, the screw 11 is used by choosing a master bolt13 having the correct length shank 19 for the particular rotationalmolding machine 1. The master bolt shank is passed through a clearancehole in the top spider 3 until the head 21 contacts the spider. The tippilot 29 is inserted into the bore 25 of the master bolt 13 until thetip shoulder 31 abuts the master bolt end 23. In that condition, thecross holes 27 and 33 are coplanar. Rotation of the master bolt and tiprelative to each other enables the cross holes to become axially alignedfor inserting the pin 17 through them. The screw 11 is thus assembled,and it is captured in the top spider by the cooperation of the head 21and pin 17.

With the screws 11 captured in the spider 3, the screws are used on therotational molding machine 1 in generally the same way as conventionalscrews. That is, the threads 34 are engagable with the threads ofassociated receivers 7 that are part of the bottom spider 5. In theparticular example of molding machine 1 shown, each receiver 7 is insideand is connected by a pin 43 to an associated short tube 45. In turn,the tube 45 is welded to the bottom spider. At the beginning of amolding cycle, power wrenches, not illustrated, drive the screw heads 21to fully engage the screws 11 with their respective receivers 7 in amanner that closes the molds 6 and 6A. At the end of the cycle, thepower wrenches reverse the screws to completely disengage the threads 34from the receivers. The spiders are separated, thereby separating themold along the parting line 47. The master bolt head 21 and the pin 17cooperate to hold the screw from falling out of the top spider when thespiders are separated. After the workpieces have been removed from themolds, the spiders are brought together, and the screws are again driveninto full engagement with their associated receivers. The guides 37 onthe tips 15 facilitate initial entry of the tip threads back into thereceivers.

When the tip threads 34 of a screw with replaceable threads 11 becomeworn, the pin 17 is removed. The tip 15 is then immediately separablefrom the master bolt 13 and discarded. A new tip is then inserted intoand pinned to the master bolt. It is thus not necessary to replace theentire screw 11 when the threads wear.

Similarly, when a tip thread 34 seizes to its receiver 7, removing thepins 17 and 43 enables the tip 15 and receiver to be removed as a unitfrom the spiders 3 and 5 without requiring any flame cutting of thescrew or other non-productive effort. A new tip and receiver can beinstalled quickly and with minimal interruption of production. Inaddition, the cost of the master bolt 13 is saved.

In summary, the results and advantages of rotational molding equipmentcan now be more fully realized. The screw with replaceable threads 11enables high production to be maintained from the equipment despite thesevere service to which they are exposed. This desirable result comesfrom using the combined functions of the replaceable tip 15 and the pin17. The pin and master bolt head 21 cooperate to prevent the screw fromfalling out of a spider 3 during production. The tip is easily andquickly removable from the master bolt 13 when the tip is worn or seizedto a spider receiver 7 merely by removing the pin from the cross holes27 and 23. A replacement tip is equally easily assembled to the masterbolt by inserting the tip pilot 29 into the master bolt bore 25 untilthe tip shoulder 31 abuts the master bolt end 23. Aligning the crossholes enables reinsertion of the pin.

It will also be recognized that in addition to the superior performanceof the invention, its construction is such as to cost but little morethan traditional screws. The great increase in productivity available torotational molding equipment resulting from the screw with replaceablethreads amply justifies its cost.

Thus, it is apparent that there has been provided, in accordance withthe invention, a screw with replaceable threads that fully satisfies theaims and advantages set forth above. While the invention has beendescribed in conjunction with specific embodiments thereof, it isevident that many alternatives, modifications, and variations will beapparent to those skilled in the art in light of the foregoingdescription. Accordingly, it is intended to embrace all suchalternatives, modifications, and variations of the invention as fallwithin the spirit and broad scope of the appended claims.

I claim:
 1. A method of repairing bolts that pass through top and bottommold carrying spiders of a rotational molding machine and that engageand disengage respective receivers on the bottom spider and thereby joinand release, respectively, the spiders to each other in a manner thatwears threads on the bolts and that causes the bolts to seize in thereceivers comprising the steps of:a. passing the shanks of a pluralityof master bolts through associated holes in the top spider that arealigned with respective receivers in the bottom spider; b. assemblingfirst threaded tips one to each master bolt shank; c. selectivelyturning the master bolts and engaging and disengaging the tips with theassociated receivers; d. removing a first tip from the associated masterbolt when the threads of the first tip have worn; and e. assembling asecond tip to the master bolt.
 2. The method of claim 1 wherein the stepof assembling the first threaded tip to the master bolt comprises thesteps of:a. removably inserting a pin through the master bolt shank andthe tip; and b. capturing the master bolt on the top spider by means ofthe pin.
 3. A method of repairing bolts that pass through top and bottommold carrying spiders of a rotational molding machine and that engageand disengage respective receivers on the bottom spider and thereby joinand release, respectively, the spiders to each other in a manner thatwears threads on the bolts and that causes the bolts to seize in thereceivers comprising the steps of:a. passing the shanks of a pluralityof master bolts through associated holes in the top spider that arealigned with respective receivers in the bottom spider; b. assemblingfirst threaded tips one to each master bolt shank; c. selectivelyturning the master bolts and engaging and disengaging the tips with theassociated receivers; d. removing a first tip from a selected masterbolt and removing the associated receiver from the bottom spider as aunit when the first tip of the selected master bolt has seized to theassociated receiver; e. assembling a second tip to the selected masterbolt; and f. connecting a new receiver associated with the second tip tothe bottom spider.